I'm a vegetarian, and really enjoyed my meal at Asherkus in Norway. They have a very nice spread of hors d'oeurves that anyone can help themselves to, and there is usually at least one vegetarian item on the menu. If you have them note that you are vegetarian on the ADR, the chef will come out and ask if there's anything different that he can make for you if the dish on the menu is not appealing (at least that was my experience). I went with two others and they made three different veggie dishes. If you're going with your mom you may enjoy the princesses too!
The vegetarian lasagna at Chefs de France is very, very good, and the mac n cheese is vegetarian as well. The setting here was very nice, and the service was excellent.
I would not suggest San Angel Inn. When I went in May, there were only two vegetarian options, and I was not very impressed with the meal that I got. I would strongly suggest the CS in Mexico over the TS.
Most places are very accommodating if you give notice, and can usually come up with some better options if the menu is limited.